In a conventional cathode ray tube, particulate cathodoluminescent phosphor material is provided as a deposit on the internal face-plate surface of an evacuated lead-glass envelope. The rear surface of the phosphor deposit is coated with conductive material, which latter provides the tube anode. When the phosphor is bombarded by electrons, light is emitted. This light is scattered, however, by neighbouring phosphor particles. For high definition applications, fine particle phosphors are used. The ultimate definition, however, is limited by particle-scattering, and the tubes are far from the ideal required for photo-typesetting and tele-cine applications. Furthermore, under high intensity bombardment, phosphor material can become depleted, and the glass can melt, reform, and phosphor can become embedded in the glass at the high localised temperatures that result from electron absorbtion, ie under extreme screen loadings "burning" of the lead glass tube faceplate limits the useful life of tubes intended for high intensity application.
For at least a decade now, cathode ray tube design has been under scrutiny, with a view to eliminating the glass face-plate part of the tube and replacing it with a face-plate of solid phosphor material. A major problem has been the provision of an effective vacuum tight seal between the solid face-plate and lead-glass envelope. In one instance recently reported (Appl Phys Lett Vol 37 No5 pp 471-2, 1980 ) this problem has been avoided by using tube material other than lead-glass. The high intensity projection television tube, described therein, comprises a face-plate of yttrium aluminium garnet (YAG) single crystal and a tube body of high density sintered alumina. The face-plate is sealed to the tube body by thermocompression bonding using aluminium as the sealant material. For this choice of alumina and garnet materials the expansion properties of both the body and the attached face-plate are well matched. However, this approach to the problem is complex, expensive, and requires specialist equipment for tube manufacture.